Rear projection screens are increasingly being used for projection displays in televisions, computer monitors, and other types of displays, for example. In general, rear projection display screens need to have high transmittance while minimizing ambient reflections to ensure adequate brightness and contrast. These characteristics are particularly necessary for displays viewed in high ambients, such as in avionics applications.
Beaded screens provide high transmittance with minimal ambient reflections. Beaded screen suffer, however, from a variety of visual artifacts, including sparkle or speckle. Such defects can occur as a result of local bead defects, or when light transmitted by a particular portion of the screen is mutually coherent with light transmitted by a neighboring portion of the screen. The mutually coherent light from neighboring portions of the screen interferes as it propagates away from the screen. A viewer's eye integrates such interference over the whole screen, with the result being that the viewer sees a number of bright spots across the screen. Both causes are referred to herein as speckle. The speckle decreases the viewability of the image projected from the screen.
A measure of speckle is the speckle contrast, which is defined as the ratio of the standard deviation of the pixel brightness over the average pixel brightness. If the speckle contrast of a screen is above a certain level, the speckle in the viewed image can be significantly distracting to the viewer. Accordingly, it is important to reduce the speckle contrast to a level acceptable to the viewer while substantially preserving other screen characteristics.
When screens are designed to enhance one or more particular characteristic, it is often found that other characteristics are degraded, or the cost of the screen assembly, or its complexity, is increased. For example, the introduction a component to the screen for reducing glare may adversely affect one of the other screen characteristics, such as gain, resolution or speckle. Ideally, measures taken to reduce speckle should affect the other screen characteristics as little as possible.
One proposed solution to the problem of speckle is described in U.S. Pat. No. 6,466,368 entitled “Rear Projection Screen with Reduced Speckle.” However, this proposed solution has disadvantages. For example, it describes use of a diffuser layer on the front of the screen from the viewer side. A diffuser layer on the front of the screen affects the high ambient contrast of the display because backscatter of incident ambient light from the diffuser layer significantly increases the reflectivity of the screen assembly.
Thus, there is a need to reduce speckle in projection screens while substantially maintaining the other screen characteristics. Further, there is a need to reduce speckle without adversely affecting contrast. Even further, there is a need for projection screens configured for the unique aspects of high ambients experienced in avionics applications.